A water absorbent resin (SAP/Super Absorbent Polymer) is a water-swelling and water-insoluble gelatinized polymer and is heavily used mainly for disposable purposes, i.e., for absorbent articles such as a disposable diaper and a sanitary napkin and for an agriculture/horticulture water retention agent, an industrial waterproofing material, and the like. As a raw material for such a hydrophilic polymer or water absorbent resin, there have been proposed many kinds of monomers. Of these raw materials, a polyacrylic acid (salt)-based water absorbent resin in which acrylic acid and a salt thereof are used as monomers or a combination of a polyacrylic acid-based water absorbent resin and a polyacrylic acid salt-based water absorbent resin is particularly industrially most widely used from the viewpoint of its high water absorption performance.
According to advancement of a disposable diaper which is the main purpose of use of the water absorbent resin, the water absorbent resin is required to have various functions (physical properties). Specific examples of the physical properties of the water absorbent resin include not only merely a high water absorption capacity but also a gel strength, a water soluble component, a water absorbing speed, a water absorption capacity under load, a liquid permeability, a particle size distribution, a urine resistance, an antibacterial property, an impact resistance (damage resistance), a powder fluidity, a deodorizing property, a coloration resistance (whiteness), a dust suppression property, and the like.
Of the physical properties described above, the liquid permeability is considered to be a more important physical property according to an increase in used amount (e.g., 50 wt % or more) of a water absorbent resin in a disposable diaper. Further, in addition to the liquid permeability, the water absorbing speed is considered to be fundamental physical properties of a water absorbent resin. Under the circumstances, there have been studied techniques for improving the liquid permeability of, preferably both the liquid permeability and the water absorbing speed of a water absorbent resin.
In addition, other than the physical properties described above, a heat retaining property of a water absorbent resin has gained attention according to an increase in used amount of a water absorbent resin in a disposable diaper (e.g., 40 mass % or more in which a water absorbent resin powder is contained based on a total mass of the water absorbent resin powder and a fibrous material), or according to use of disposable diapers in regions in various climates.
Patent Literature 1 studies a method for producing a polyacrylic acid (salt)-based water absorbent resin powder having both a higher liquid permeability and a higher water absorbing speed. Specifically, Patent Literature 1 studies control of a gel grinding step, a drying step, and a surface treatment step that are included in a process for producing a polyacrylic acid (salt)-based water absorbent resin powder.